This .dll should be in your Windows\System directory. Search for it on disk and when you find it, copy it into that directory. As long as that directory is in your path (and it should be), then you should have no further problems.
NI-488.2 version 2.6 is written for DOS/WIN 16 operating systems. You may want to remove that driver and install NI-488.2 version 1.6 or 1.7 instead. Here is a link to the download site: http://digital.ni.com/softlib.nsf/websearch/E10418C4F6CFDCC186256BA600644D77?opendocument&node=132060_US
Heather S. Applications Engineer National Instruments